UNIVERSITI PUTRA MALAYSIA

OMAR SULIMAN ZAROOG

FK 2011 23

RESIDUAL STRESS RELAXATION OF SHOT- PEENED 2024-T351 ALUMINIUM ALLOY

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RESIDUAL STRESS RELAXATION OF SHOT-PEENED 2024-T351

ALUMINIUM ALLOY

By

OMAR SULIMAN ZAROOG

Thesis Submitted to the School of Graduate Studies, Universiti Putra Malaysia, in

Fulfilment of the Requirements for the Degree of Doctor of Philosophy

January 2011

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DEDICATION

To

My family members especially my beloved wife, my children and my ever -encouraging

father for his love

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Abstract of thesis presented to Universiti Putra Malaysia in fulfilment of the requirement

for the degree of Doctor of Philosophy

RESIDUAL STRESS RELAXATION OF SHOT-PEENED 2024-T351

ALUMINIUM ALLOY

By

OMAR SULIMAN ZAROOG

January 2011

Chairman: Associate Prof. Aidy Ali, PhD

Faculty: Engineering

Near surface tensile residual stresses tend to accelerate the initiation and growth phases of

the fatigue process while compressive residual stresses close to a surface may prolong

fatigue life and consider as beneficial residual stresses. Shot peening process used to

induce beneficial compressive residual stress in materials. However, the residual stresses

may relax due to thermal, static mechanical load and cyclic load. Even with partial

relaxation, there found a beneficial effect of compressive residual stress on fatigue life.

The problem is how much is the relaxation? To answer this question a 2024 T351

aluminium alloy specimens were shot peened into three shot peening intensities 0.0054 A,

0.0067 A and 0.009 A. The cyclic test for the two loads magnitude, 15.5 kN and 30 kN,

was performed for the 1, 2, 10, 1000 and 10000 cycles. The initial residual stress and

residual stress as well as cold work after each cyclic load were measured for the three

shot peening intensities and for the two magnitudes of loads using X-ray diffraction

method. Initial and after microhardness of each cyclic load were measured for the three

shot peening intensities.

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The results showed that the most relaxation of the initial residual stress took place in the

first cycle; the initial residual stress was relaxed by 46% after first cycle in the load 30 kN

and shot peening intensity of 0.0054 A. Relaxation of residual stresses occurred within

first loading cycles were increasing with increasing loading stress amplitude and due to

quasi-static relaxation effects. The residual stress after the first cycle found to relax

depends on the load amplitude. The maximum relaxation found is 54% of the initial

residual stress in the shot peen intensity of 0.0054 A after 10000 cycles for the load of 30

kN. The changed in the relaxation percentages of all specimens from 10 cyclic load to

10000 cyclic load is in the range of 5-8% of the initial residual stress. Microhardnesses

were found to decrease depending on the load amplitude. A load of 30 kN made

microhardness in the specimens decrease more than the 15.5 kN load. The microhardness

was reduced by 39%, given a shot peen intensity of 0.009 A after 10000 cycles under a

load of 30 kN. From observations and results, empirical equations to estimate the residual

stress relaxation were proposed. The equations incorporated the number of cycles and

cold work to predict the amount of residual stress relaxation. Finally, the results of the

estimation are in a good agreement with experimental data.

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Abstrak tesis yang dikemukakan kepada Senat Universiti Putra Malaysia sebagai

memenuhi keperluan untuk ijazah Doktor Falsafah

RELAKSASl STRES SISA OF SHOT-PEENED 2024-T351 ALUMINIUM ALOI

Oleh

OMAR SULIMAN ZAROOG

January 2011

Pengerusi: Profesor Madya Aidy Ali, PhD

Fakulti: Kejuruteraan

Tegasan-tegasan lebihan berdekatan permukaan cenderung untuk memacu fasa-fasa

permulaan and pertumbuhan dalam proses fatig dan tegasan-tegasan lebihan mampatan

berdekatan sesuatu permukaan mungkin akan memanjangkan jangkahayat fatig dan

diandaikan sebagai tegasan-tegasan lebihan yang bermanafaat. Proses ‘shot peening’ telah

digunakan untuk menghasilkan tegasan lebihan mampatan di dalam bahan tersebut.

Walau bagaimana pun, tegasan-tegasan lebihan tersebut berkemungkinan akan berada di

dalam keadaan rehat disebabkan oleh suhu, beban mekanikal statik, dan juga beban

berputar. Walaupun dengan keadaan separa rehat, adalah didapati terdapat kesan yang

bermanafaat hasil dari tegasan lebihan mampatan terhadap jangkahayat fatig.

Persoalannya, berapa banyak jumlah keadaan rehat tersebut? Bagi menjawab persoalan

ini, spesimen aluminum aloi 2024 T351 telah di kenakan proses ‘shot peening’

berintensiti 0.0054 A, 0067 A dan 0.009 A. Ujian putaran untuk bagi dua magnitud beban

iaitu 15.5 dan 30 kN, telah dijalankan untuk 1, 2, 10, 1000 dan 10000 putaran. Tegasan

lebihan permulaan dan juga tegasan kerja sejuk selepas setiap beban putaran telah diukur

untuk tiga intensiti ‘shot peening’ dan untuk dua magnitud beban dengan menggunakan

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kaedah ‘X-ray diffraction’. Bacaan mikro-kekerasan untuk setiap beban putaran telah

diambil untuk tiga intensiti ‘shot peening’. Keputusan telah menunjukkan yang

kebanyakkan keadaan rehat pada tegasan lebihan permulaan berada pada permulaan

kitaran; tegasan lebihan permulaan telah direhatkan sebanyak 46% selepas putaran

pertama pada beban 30 kN dan intensiti ‘shot peening’ 0.0054 A. Kerehatan tegasan

lebihan telah berlaku dalam masa beban putaran pertama dan bertambah dengan

pertambahan amplitud bebanan tegasan dan juga disebabkan oleh kesan-kesan kerehatan

quasi-statik. Kerehatan tegasan lebihan selepas putaran pertama didapati bergantung

kepada amplitud bebanan. Kerehatan maksima adalah 54% dari tegasan lebihan

permulaan pada intensiti ‘shot peening’ 0.0054 A selepas 10000 beban putaran bagi

bebanan sebanyak 30 kN. Perubahan di dalam peratusan kerehatan untuk semua spesimen

daripada 10 ke 10000 beban putaran adalah di dalam julat 5 – 8% daripada tegasan

lebihan awal. Mikro-kekerasan didapati telah mengurang, bergantung kepada amplitud

bebanan. Beban sebanyak 30 kN telah menyebabkan bacaan mikro-kekerasan

berkurangan sebanyak 15.5 kN bebanan. Bacaan mikro-kekerasan telah berkurangan

sebanyak 39%, pada intensiti ‘shot peening’ 0.009 A selepas 10000 putaran pada beban

30 kN. Daripada pemerhatian dan hasil keputusan, persamaan-persamaan empirikal untuk

menganggarkan kerehatan tegasan lebihan telah dicadangkan. Persamaan-persamaan

tersebut mengambilkira jumlah putaran dan juga kerja sejuk bagi menganggarkan jumlah

kerehatan tegasan lebihan tersebut. Adalah didapati, hasil keputusan anggaran tersebut

menunjukkan persamaan dan di dalam julat yang boleh diterima jika dibandingkan

dengan data-data dari ujikaji.

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ACKNOWLEDGEMENTS

I wish to express my deepest gratitude to the numerous people who have walked with me

along the journey of this thesis. First and foremost I would like to express my deep

gratefulness to my supervisor Dr. Aidy Ali for his kind assistance, support, critical

advice, encouragement, suggestions and direction throughout my research and

preparation of this thesis. Many ideas originated in our frequent discussion and his

constant support and patience over the years have been of invaluable help.

I also wish to extend my sincere gratitude and appreciation to my co-supervisor, Professor

Ir. Dr. Barkawi Bin Sahari for his guidance, patience, understanding, encouragement and

supervision throughout the course of the study until the completion of this thesis. I truly

admire him for his openness, honesty and sincerity and appreciate the time that he

devoted in advising me and showing me the proper directions to carry this research.

I would also like to express my gratitude towards my co-supervisor Dr. Rizal Zahari for

his supervision, helpful advice and fruitful discussion that made an invaluable

contribution to this dissertation.

Last but not the least, my heart-full gratitude and love to my wife Mezahir, my children

(Soliman, Ahmed, Amgad and Asgad), my father, my (late) mother, my brothers and

sisters, my cousin Sulieman Elhory, Dr. Siddig Ibrahim, Mohammed Ibrahim, Fatah

Algoul, Dr. Ahmed Hussien, Wadah Othman, and siblings whose unconditional support

and love has made this dream come true to me.

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This thesis was submitted to the Senate of Universiti Putra Malaysia and has been

accepted in fulfilment of the requirements for the degree of Doctor of Philosophy.

Members of the Supervisory Committee were as follows:

Aidy Ali, PhD

Associate Prof. Dr.

Faculty of Engineering

Universiti Putra Malaysia

(Chairman)

Barkawi Bin Sahari, PhD

Professor, Ir.

Faculty of Engineering

Universiti Putra Malaysia

(Member)

Rizal Zahari, PhD

Senior Lecturer

Faculty of Engineering

Universiti Putra Malaysia

(Member)

HASANAH MOHD GHAZALI, PhD

Professor and Dean

School of Graduate Studies

Universiti Putra Malaysia

Date:

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DECLARATION

I declare that the thesis is based on my original work except for quotations and citations

which have been duly acknowledged. I also declare that it has not been previously and it

not concurrently submitted for any other degree at Universiti Putra Malaysia or other

institutions.

Omar Suliman Zaroog

Date: 25 January 2011

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TABLE OF CONTENTS

Page

DEDICATION II

ABSTRACT III

ABSTRAK V

ACKNOWLEDGEMENTS VII

APPROVA VIII

DECLARATION X

TABLE OF CONTENTS XI

CHAPTER

1 INTRODUCTION

1.0 Aluminium 1

1.1 Wrought aluminium alloys 2

1.2 Classification of aluminium alloys 3

1.3 Temper designation of aluminium alloys 3

1.4 2000 series aluminium alloys 4

1.5 Problem statement 7

1.6 Objectives 8

1.7 Thesis layout 8

2 LTERATURE REVIEW

2.0 Shot peening (SP) an engineering surface treatment 10

2.1 Brief history of shot peening 10

2.2 The mechanism of shot peening 13

2.3 Shot peening nomenclature and control parameters 18

2.3.1 Shot peening media 18

2.3.2 Incidence angle 20

2.3.3 Saturation condition 21

2.3.4 Intensity 22

2.3.5 Coverage 23

2.3.6 Velocity 24

2.4 Shot peening effects and fatigue 25

2.4.1 Microstructure 27

2.4.2 Topography (surface roughness) 28

2.5 Summary for shot peening 29

2.6 Residual stresses 30

2.6.1 Difinition of residual stress 30

2.6.2 Origin of residual stresses 31

2.7 Effects of residual stress 34

2.8 Incorporating the concept of residual stress into the design 37

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2.9 Relaxation of residual stresses 38

2.10 Summary for residual stresses 41

2.11 Measuring of residual stresses 43

2.12 Destructive methods 43

2.12.1 Curvature 43

2.12.2 Crack compliance methods 45

2.12.3 Hole drilling 45

2.13 Non destructives methods 48

2.13.1 Magnetic 48

2.13.2 Ultrasonic method 50

2.13.3 Diffraction methods 51

2.14 Summary of residual stress measuring techniques 55

2.14.1 XRD sources of Errors 55

2.14.2 Uncertainty of hole drilling method 56

2.15 Modelling of residual stress relaxation 59

2.16 Summary of modelling of residual stress relaxation 62

3 METHODOLOGY

3.0 Introduction 64

3.1 Materials 64

3.2 Specimens cutting 66

3.3 Shot peening 67

3.4 Cyclic load test 67

3.5 Microhardness measurements 69

3.6 Residual stress measurements 70

3.7 Cold work % measurements 71

3.8 Modelling of residual stress relaxation 71

4 RELAXATION OF COMPRESSIVE RESIDUAL STRESS. PART 1:

RELAXATION OF STAGE I 75

Article 1 75

Acceptance letter 84

Copyright permission 85

5 RELAXATION OF COMPRESSIVE RESIDUAL STRESSES. PART

2: RELAXATION OF STAGE 2 86

Article 2 86

Acceptance letter 101

Copyright permission 102

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6 RESIDUAL STRESS RELAXATION AND SURFACE HARDNESS OF

2024-T351 ALUMINIUM ALLOY 103

Article 3 103

Acceptance letter 129

Copyright permission 130

7 MODELING OF RESIDUAL STRESS RELAXATION OF FATIGUE

2024-T351 ALUMINIUM ALLOY 131

Article 4 131

Acceptance letter 157

Copyright permission 158

8 CONCLUSION AND RECOMMENDATIONS

8.1 Conclusion 159

8.2 Recommendations 161

REFERENCES 162

BIODATA OF STUDENT 174

STUDENT’S PUBLICATIONS 175